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Rabi oscillation, originally proposed in nuclear magnetic resonance, is a well-known phenomenon associated with a driven two-level system. Although magnetic fields typically can bring about chirality into unusual phenomena such as chiral edge states in the quantum Hall effect, it is not clear if chirality exists in Rabi oscillations. Here we unveil the intrinsic chirality carried by the phase in a Rabi problem. For opposite detuning of the driving field, the phase evolution of the probability amplitude exhibits a mirror symmetry. Consequently, constructive or destructive interference of two off-resonant Rabi processes under different initial conditions is level dependent and symmetry protected. Experimentally, we demonstrate such features in a photonic setting with adjustable detuning, yet our results may prove pertinent to the study of similar phenomena in other driven two-level systems beyond photonics.